Injection molding of plastic materials is well known and widely practiced as a means of manufacturing an ever-increasing diversity of plastic components for industrial and consumer use. During the last two decades, versions of the process globally referred to as “gas assisted molding” have been developed and used to overcome some of the problems inherent in conventional molding, and to reduce costs and improve the quality of the final products.
In conventional gas molding, a gas, such as nitrogen, is injected into the molten plastic material after it has entered the mold. The low viscosity gas flows into the paths of least resistance within the more viscous plastic, thereby forming hollow channels within the plastic. The process is particularly beneficial for thick section moldings, such as handles, and weight savings of up to 45% or more can result. Also, the molding time cycles can be substantially reduced. In multi-section moldings, the injected gas tends to flow into the thicker sections, again forming hollow continuous channels through which pressure may be transmitted through the medium of the gas. This adds to the scope of the designer and removes some of the design restrictions of conventional molding.
One conventional gas assisted plastic molding process partially fills the mold cavity with an accurately controlled shot volume of plastic. Gas is then injected in order to continue the flow of plastic so that the cavity is filled with plastic and gas. The gas is used to exert outward pressure on the plastic material forcing it against the mold cavity surfaces, thereby achieving a good replication of the mold surface on the plastic molded surface. After the plastic has solidified, the gas pressure is reduced, the gas is exhausted to atmosphere, and the mold is opened and the part ejected. This is sometimes referred to as a “short shot” process.
In another method, the mold cavity is completely or substantially filled with plastic material and then instead of injecting or packing more plastic into the cavity to compensate for the volumetric shrinkage of the plastic as it cools and solidifies, gas is injected into the plastic so that the gas expansion compensates for the plastic contraction. In practice, the initial gas penetration will continue to expand during the cooling cycle while the plastic is shrinking in volume. This is sometimes referred to as a “full shot” process.
In the “full shot” process, it is sometimes difficult to achieve sufficient gas penetration along intended gas channels because there is insufficient volumetric shrinkage of the plastic to provide space for the gas. In such cases, a method of enabling some plastic to outflow from the mold cavity into overflow wells or “secondary” cavities is helpful in providing space for the gas expansion.
In the “short shot” method, some moldings may also be difficult to fill with plastic and gas to the extremities of the molded article cavity. If the shot volume is too little, the gas may break through the leading edge of the plastic material during filling, thereby losing control of the gas. If the shot volume is too high, the gas will not reach the extremities of the cavity. Therefore thick section moldings using the “short shot” process can also benefit from an additional displacement of plastic from the article cavity into an overflow cavity.
A method of at least partially filling the cavity before injection of the gas is described in U.S. Pat. No. 5,098,637. However, in order to use the method of this patent successfully, it is necessary to accurately control the shot volume for both “short shot” and “full shot” methods, because there is no resistance to prevent the plastic from flowing into and filling the overflow cavity before the gas is injected.
In the “short shot” process, the flow of plastic is temporarily stopped at the end of the filling sequence, and then typically there is a delay of up to five seconds before injection of the gas urges the plastic forward to complete the filling of the article cavity with plastic and gas and the overflow cavity with plastic. In the “full shot” process, the mold cavity is filled with, or substantially filled with, molten plastic and the gas is injected to compensate for the volumetric shrinkage of the plastic and to displace plastic into the overflow cavity. In both cases, it is not feasible to apply “packing pressure” by the molding machine because there is nothing to restrain the further flow of plastic. In the “short shot” process, or when nearly filling the cavity in a “full shot” process, there may remain unsightly visible hesitation lines or marks on the molding surface at the position of the first plastic injection.
Another method is disclosed in Japanese Patent Application No. 50-74660, where shut-off valves in runners connect the product cavity and “secondary cavities.” In this application, the mold cavity is filled with thermoplastic resin and then a core resin or gas is injected into the cavity while the thermoplastic resin in the mold is expelled from the mold cavity. After the thermoplastic resin fills the mold cavity, the core resin or gas is injected while the resin in the mold cavity is expelled.